easy fix. it is your "resistor block" block of two(?) resistors behind the glove box that control the current for each setting. each setting uses a different combo of the resistors to set the current. lose one of the resistors and some of the speeds won't work. It is a little box about 2" across with a big 4 or 5 wire plug going into it. Unplug and if your fan no longer works at all, you have found it. Your blower motor should be fine. Not really sure where the best place to get a new one would be.

edit* just noticed you got a 91 and everything i was said was for a 94+, should be a similar layout though.